LIGO/Virgo/KAGRA S251108fi
GCN Circular 42632
Rosa L. Becerra (UNAM), Alan M. Watson (UNAM), Camila Angulo Valdez (UNAM), Nat Butler (ASU), Simone Dichiara (Penn State University), Tsvetelina Dimitrova (ASU), Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Océlotl López (UNAM), Margarita Pereyra (UNAM) and Eleonora Troja (U Roma) report:
We observe the field of the gravitational wave candidate S251108fi (LVKC, GCN Circ. 42624) with the DDOTI/OAN wide-field imager at the Observatorio Astronómico Nacional on the Sierra of San Pedro Martir (http://ddoti.astroscu.unam.mx) on the night of 2025-11-10 UTC.
DDOTI observed from 02:22 UTC to 04:35 UTC (T+31.4 h to T+33.6 h after the trigger), with a total exposure time of 72 minutes, pointing the region reported by the IceCube Collaboration (GCN Circ. 42630).
Comparing our observations with the USNO-B1 and Pan-STARRS PS1 DR2 catalogues, we identify an uncatalogued source at:
RA = 00:57:11.36 (14.2973 d)
DEC= +07:39:07.1 (7.6520 d)
with a preliminary magnitude AB magnitude of:
w = 20.11 +/- 0.11.
This source lies outside the IceCube region but within the 90% credible region reported by the LVK Collaboration (GCN Circ. 42628). It is located approximately 4" from a known galaxy with a photometric redshift of z = 0.214 ± 0.031 in the Legacy Survey DR10 (Dey et al. 2019). If they are associated, such a redshift would make a connection with the GW candidate unlikely.
However, further observations and analysis are ongoing.
This value is not corrected for the Galactic extinction.
We thank the staff of the Observatorio Astronómico Nacional on the Sierra of San Pedro Mártir.
GCN Circular 42630
IceCube Collaboration (http://icecube.wisc.edu/) reports:
Searches for track-like muon neutrino events detected by IceCube consistent with the sky localization of gravitational-wave candidate S251108fi (GCN 42628) in a time range of 1000 seconds centered on the alert event time (2025-11-08 19:05:08.506 UTC to 2025-11-08 19:21:48.506 UTC) have been performed [1,2]. During this time period IceCube was collecting good quality data. Two hypothesis tests were conducted. The first search is a maximum likelihood analysis which searches for a generic point-like neutrino source coincident with the given GW skymap. The second uses a Bayesian approach to quantify the joint GW + neutrino event significance, which assumes a binary merger scenario and accounts for known astrophysical priors, such as GW source distance, in the significance estimate [3].
One track-like event is found in spatial and temporal coincidence with the gravitational-wave candidate S251108fi calculated from the map circulated in the 4-Update notice. This represents an overall p-value of 0.0076 from the generic transient search and an overall p-value of 0.0421 for the Bayesian search. These p-values measure the consistency of the observed track-like events with the known atmospheric backgrounds for this single map (not trials corrected for multiple GW events). The most probable multi-messenger source direction based on the neutrinos and GW skymap is RA 15.12, Dec 9.06 degrees.
The reported p-values can differ due to the estimated distance of the GW candidate. The distance is used as a prior in the Bayesian binary merger search, while it is not taken into account in the generic transient point-like source search. The false alarm rate of these coincidences can be obtained by multiplying the p-values with their corresponding GW trigger rates. Further details are available at https://gcn.nasa.gov/missions/icecube. Additional details and updates will be posted at https://roc.icecube.wisc.edu/public/LvkNuTrackSearch/.
Properties of the coincident event are shown below.
| dt(s) | RA(deg) | Dec(deg) | Angular uncertainty(deg) | p-value(generic transient) | p-value(Bayesian) |
|---|---|---|---|---|---|
| -242.48 | 15.01 | 9.06 | 1.13 | 0.0076 | 0.0421 |
where: dt = Time of track event minus time of GW trigger (sec). Angular uncertainty = Angular uncertainty of track event: the radius of a circle representing 90% CL containment by area. p-value = the p-value for this specific track event from each search. Event p-values are provided when the per-event p-value is less than 0.1 in either search.
The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc@icecube.wisc.edu
[1] M. G. Aartsen et al 2020 ApJL 898 L10 [2] Abbasi et al. Astrophys.J. 944 (2023) 1, 80 [3] I. Bartos et al. 2019 Phys. Rev. D 100, 083017
GCN Circular 42628
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:
We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S251108fi (GCN Circular 42624). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:
https://gracedb.ligo.org/superevents/S251108fi
For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 528 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2972 +/- 957 Mpc (a posteriori mean +/- standard deviation).
For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.
[1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040
GCN Circular 42624
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:
We identified the compact binary merger candidate S251108fi during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-11-08 19:13:28.506 UTC (GPS time: 1446664426.506). The candidate was found by the Aframe [1], cWB [2], cWB BBH [3], GstLAL [4], MBTA [5], and PyCBC Live [6] analysis pipelines.
S251108fi is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.3e-12 Hz, or about one in 1e4 years. The event's properties can be found at this URL:
https://gracedb.ligo.org/superevents/S251108fi
The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<1%).
Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [7] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [7] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.
The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.
Two sky maps are available at this time and can be retrieved from the GraceDB event page:
* bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [8], distributed via GCN and SCiMMA notices about 25 seconds after the candidate event time.
* bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [8], distributed via GCN and SCiMMA notices about 4 minutes after the candidate event time.
The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 1524 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 3537 +/- 1032 Mpc (a posteriori mean +/- standard deviation).
For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.
[1] Marx et al. PRD 111, 042010 (2025) doi:10.1103/PhysRevD.111.042010
[2] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
[3] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
[4] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
[5] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
[6] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
[7] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
[8] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013